Broadly, the invention disclosed herein pertains to a method and apparatus for controlling input/output transfers between a computer and a number of peripheral terminal units. More particularly the subject invention relates to such a method and apparatus for use in conjunction with apparatus of a program controlled interrupt system as disclosed in U.S. Pat. No. 3,728,693 issued to John Arthur Macker and William Francis Keenan on April 17, 1973 and assigned to the present assignee. As disclosed in that patent, I/O operations may be accomplished under control of a number of programmatically established descriptors stored in a memory system cooperating with a computer processor. The ability to programmatically link a number of descriptors provides the ability to chain input/output operations, and thereby avoid processor intervention during each I/O operation. This method of I/O control proves to have drawbacks when it is desired to rapidly poll peripherals. Particularly, the requirement that the processor intervene after each I/O operation proves particularly time-consuming and wasteful of processor power.